1. Field of the Invention
The present invention relates to a power conversion apparatus for converting a DC power supply from DC power to AC power, or converting an AC power supply from AC power to DC power, and more particularly to a power conversion apparatus capable of easily providing a large capacity (high voltage, large current).
2. Description of the Related Art
Power conversion apparatuses for converting a direct current to an alternating current or an alternating current to a direct current are roughly classified into voltage-type power conversion apparatuses and current-type power conversion apparatuses (see, for example, non-patent document 1 recited below).
The voltage-type power conversion apparatuses are represented by voltage-type inverters, which turn on and off a DC voltage and DC current using a self-commutated device, thereby converting the DC power of a DC power supply to AC power, or the AC power of an AC power supply to DC power.
When the self-commutated device is in the off state, the current is abruptly reduced from, for example, about 1000 A/μs to zero, and the voltage is abruptly increased, thereby generating a surge voltage by the inductance of the circuit. In particular, to realize a large capacity (high voltage, large current), the circuit must be made longer because of high voltage, thereby increasing the inductance and current. Accordingly, it is difficult to realize a large capacity.
The current-type power conversion apparatuses are represented by a thyristor converter for converting the AC power of an AC power supply to a DC power. The ON-timing of the thyristor is controlled in synchronism with the cycle of the AC power supply voltage, whereby the current of the thyristor is made zero by the power supply commutation utilizing the AC power supply voltage, thereby turning off the thyristor.
In this case, although the thyristor can be controlled only when it is in the on state, it has a simple structure and can easily realize high voltage and large current. Further, in a thyristor converter, the rate of change in current when the apparatus is on and off state is about 10 A/μs, and if the apparatus is used as a power conversion apparatus, it can relatively easily provide a large capacity (high voltage, large current).
A load commutation type inverter (also called an LCI) is another typical example of the current-type power conversion apparatus for converting DC power to AC power. This inverter is similar in base structure to the thyristor converter. In this case, however, a synchronous motor is connected as an AC load to the inverter, and the current flowing through the thyristor is made zero, i.e., the thyristor is turned off, by a load commutation based on the induced voltage of the synchronous motor.
The combination of the load commutation type inverter and synchronous motor is called a thyristor motor or thyristor commutatorless motor, which can relatively easily provide a large capacity (high voltage and large current) like the thyristor converter.
In addition, inventions utilizing a self-commutated device as a semiconductor power device in a current-type power conversion apparatus have been made so far (see, for example, the following patent documents 1 to 6).
Non-patent Document 1: “Power Electronics Circuits”, p. 137 and p. 155, Compiled by the Institute of Electrical Engineers, the Semiconductor Power Conversion System Research Committee, and Published Nov. 30, 2000 by Ohm Corporation
Patent Document 1: Jpn. Pat. Appln. KOKAI Publication No. 5-115178; FIG. 1
Patent Document 2: Jpn. Pat. Appln. KOKAI Publication No. 5-122936; FIG. 1
Patent Document 3: Jpn. Pat. Appln. KOKAI Publication No. 5-236759; FIGS. 1 and 6
Patent Document 4: Jpn. Pat. Appln. KOKAI Publication No. 5-236760; FIG. 1
Patent Document 5: Jpn. Pat. Appln. KOKAI Publication No. 6-30568; FIG. 1
Patent Document 6: Jpn. Pat. Appln. KOKAI Publication No. 7-222462; FIGS. 1 and 3